This invention relates to a novel family of metallophosphates, collectively designated AlPO-77. They are represented by the empirical formula:HwMm2+EPxSiyOz where M is a divalent framework metal such as magnesium or zinc, and E is a trivalent framework element such as aluminum or gallium.
Classes of molecular sieves include crystalline aluminophosphate, silicoaluminophosphate, or metalloaluminophosphate compositions which are microporous and which are formed from corner sharing AlO4/2 and PO4/2 tetrahedra. In 1982, Wilson et al. first reported aluminophosphate molecular sieves, the so-called AlPOs, which are microporous materials that have many of the same properties as zeolites, although they do not contain silica (See U.S. Pat. No. 4,310,440). Subsequently, charge was introduced to the neutral aluminophosphate frameworks via the substitution of SiO4/2 tetrahedra for PO4/2+ tetrahedra to produce the SAPO molecular sieves as described by Lok et al. (See U.S. Pat. No. 4,440,871). Another way to introduce framework charge to neutral aluminophosphates is to substitute [Me2+O4/2]2− tetrahedra for AlO4/2− tetrahedra, which yields the MeAPO molecular sieves (see U.S. Pat. No. 4,567,029). It is furthermore possible to introduce framework charge on AlPO-based molecular sieves via the simultaneous introduction of SiO4/2 and [M2+O4/2]2− tetrahedra to the framework, giving MeAPSO molecular sieves (See U.S. Pat. No. 4,973,785).
Numerous molecular sieves, both naturally occurring and synthetically prepared, are used in various industrial processes. Synthetically, these molecular sieves are prepared via hydrothermal synthesis employing suitable sources of Si, Al, P, and structure directing agents such as amines or organoammonium cations. The structure directing agents reside in the pores of the molecular sieve and are largely responsible for the particular structure that is ultimately formed. These species may balance the framework charge associated with silicon or other metals such as Zn or Mg in the aluminophosphate compositions, and can also serve as space fillers to stabilize the tetrahedral framework. Molecular sieves are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent molecular sieve crystal structure. Molecular sieves can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
As stated above, molecular sieves are capable of reversibly adsorbing and desorbing certain molecules depending on the adsorbate's size and the molecular sieve's internal pore structure. There are instances where it may be desirable to alter the pore structure of a given molecular sieve in order to more efficiently adsorb a certain molecule, or to more efficiently exclude a competing molecule in a given process stream. For example, the dehydration of natural gas streams requires an adsorbent to remove water vapor while excluding larger molecules such as methane, hydrogen sulfide, or carbon dioxide. One method of altering the pore structure of a molecular sieve is through ion-exchange. This is well-known in the art, and commonly applied to aluminosilicates (i.e., zeolites). By exchanging a smaller alkali metal cation with a larger alkali metal cation, the effective pore size can be reduced, and larger molecules that would compete with water for adsorption in the zeolite pores can be excluded (see, for example, U.S. Pat. Nos. 3,024,867, 4,663,052).
Another way that the adsorptive properties of a molecular sieve can be altered is through the degree of hydration of the molecular sieve. According to Bish and Carey, there is a class of zeolites that undergo topological changes in their framework connectivity upon dehydration (Bish, D. L., and Carey, J. W. (2001) “Thermal behavior of natural zeolites” In D. L. Bish and D. W. Ming, Eds., NATURAL ZEOLITES: OCCURRENCE, PROPERTIES, APPLICATIONS, vol. 45, p. 403-452. Reviews in Mineralogy and Geochemistry, Mineralogical Society of America). This topological change may be reversible or irreversible. Similar observations have been made in other aluminosilicates by Alberti and Martucci (STUD. SURF. SCI. CATAL. 155, 19-43 (2005)), as well as by Cruciani. (J. PHYS. CHEM. SOL. 67, 1973-1994 (2006). It is also known in the art that certain aluminophosphate materials can undergo topological changes upon dehydration. For example, it was observed by Keller et al. that the material AlPO-C (APC structure) has been observed to transform into both AlPO-D (APD structure) as well as AlPO-H3, depending upon the conditions of dehydration. (Keller, E. B., et al. SOLID STATE IONICS 43, 93 (1990).